Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-In HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels or as a power storage device to store surplus power such that the power can be used as needed.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a battery module having a plurality of battery cells electrically connected to each other because high output and large capacity are necessary for the middle or large-sized devices.
Preferably, the battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle or large-sized battery module. In particular, much interest is currently focused on such a pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight and the manufacturing cost of the pouch-shaped battery is low.
Meanwhile, the battery module is configured to have a structure in which a plurality of battery cells is combined. In a case in which some of the battery cells are overcharged or overdischarged, therefore, a large amount of gas is generated due to decomposition of an electrolyte with the result that a battery case of each of the battery cells swells, i.e. a so-called swelling phenomenon occurs. The swelling phenomenon further accelerates decomposition of the electrolyte while causing high pressure in the sealed battery case with the result that the battery module may explode. Furthermore, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells with the result that a fire or explosion of the battery module may occur, which may lead to a large-scale accident.
For this reason, the secondary battery is provided with a safety system, such as a protection circuit to interrupt electric current when the secondary battery is overcharged or overdischarged or when overcurrent flows in the secondary battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the secondary battery increases, or a safety vent to interrupt electric current or to exhaust gas when pressure increases due to generation of the gas. On the other hand, a multi cell type middle or large-sized battery pack, which is configured to have a multi cell structure in which a plurality of battery modules is combined, is provided with a safety system, such as a fuse, a bimetal, or a battery management system (BMS), to protect battery cells from overcharge, overdischarge, or overcurrent.
However, the safety system, such as the PTC element or the BMS, may malfunction. On the other hand, a current interrupt device (CID) system, which is mainly used as a method of interrupting electrical connection between a middle or large-sized battery pack and an external device when the middle or large-sized battery pack abnormally operates, is configured to operate only when the outermost battery modules or the outermost battery cells abnormally operate. In a case in which the outermost battery modules or the outermost battery cells normally operate, therefore, it is difficult to exhibit a desired effect.
In addition, it is necessary to inevitably provide a space through which the battery modules or the battery cells are partially exposed to the outside even in a case in which the outermost battery modules or the outermost battery cells swell with the result that it is difficult to increase the capacity per volume.
Consequently, there is a high necessity for a battery cell assembly configured to have a novel structure that is capable of fundamentally securing safety of the battery module and the battery pack while solving the above problems.